1. Field of the Invention
The present invention relates to a digital electronic device, and more specifically, to an optical storage device capable of writing to an optical disk.
2. Description of the Prior Art
Optical media is a popular data storage means having high storage density, reliable data stability, and good portability. Compact disks (CDs) and digital versatile disks (DVDs) have all but replaced traditional competing storage devices such as magnetic floppy disks and audio and video tapes. While digital devices used to read and write optical media, such as CD-ROM drives and DVD players, are complicated and highly precise, technological innovation has made these devices reliable and practical.
In the past, optical disk drives were mainly available to end users as read only devices. Most users were satisfied with simply receiving software, music, or movies on CDs or DVDs and gave little thought to using the devices as storage for themselves. Optical disk drives capable of writing were specialized and expensive pieces of equipment used mainly by media publishers to produce their products. More recently, writable optical disk drives, such as common CD-RW drives for use with computer systems, have become widely available to individuals. And as a result, the need to improve performance of these devices has increased considerably.
FIG. 1 shows a typical CD-RW drive 10 that is commonly used in computer systems. The CD-RW drive 10 is capable of reading, writing, and erasing data on a CD 12. The basic operations of CD-RW drives are well known in the art. Additionally, Van Der Zande et al. teaches operation of a writable optical drive in detail in U.S. Pat. No. 4,901,300, which is incorporated herein by reference.
A fundamental operation of the CD-RW drive 10 is an optimal power calibration (OPC) process in which the optimal recording power for a given CD is determined. The OPC process must be completed before recording to the CD can begin. The optimal recording power is determined by performing a series of recording tests to a power calibration area of the CD. A crucial value in the OPC process is the Orange Book gamma (γ) value, which relates recorded RF quality to recording power as follows:
                    γ        =                              (                                          ⅆ                m                                            ⅆ                                  P                  W                                                      )                                (                          m                              P                W                                      )                                                                (                      Eqn            .                                                  ⁢            1                    )                    
where,
m represents the modulation amplitude of the RF signal;
PW is recording power; and
dm/dPW is a differential of modulation amplitude with respect to recoding power;
An optimal or target gamma value is written to a blank CD at the time of manufacture, and is read by the optical drive when recording is to be performed. Conventionally, during the OPC process, several recording powers as tested by writing to the power calibration area of the CD. The modulation amplitude of each test power is measured, the derivative of the modulation amplitude with respect to recording power is numerically determined along with other calculations according to Eqn. 1, and a gamma curve is generated. The target power is then determined from the gamma curve referencing the target gamma value.
The previously described procedure is illustrated in FIG. 2 showing a graph of measured modulation amplitude m and gamma value γ versus recording power. When performing the OPC procedure, microprocessors and control circuitry of a conventional optical diskdrive first measure modulation amplitudes for a series of recording powers to generate the modulation amplitude curve as indicated by numeral 20 in FIG. 2 (the 4 points shown being representative). The optical disk drive then calculates a gamma curve from the measured modulation data, as indicated by numeral 22. Then, to obtain the target power the optical disk drive references the target gamma and calculates or looks up the target power. Finally, optimal write and erase powers are determined as follows:
                                                                        P                W0                            =                              ρ                ⁢                                                                  ⁢                                  P                  T                                                                                                                        P                E0                            =                              ɛ                ⁢                                                                  ⁢                                  P                  W0                                                                                        (                  Eqns          .                                          ⁢          2                )            
where,
PWO is the optimal recording power;
ρ is a recording constant;
PT is the target power;
PEO is the optimal erasing power; and
ε is an erasing constant;
The constants ρ and ε for respectively determining the optimal recording and erasing powers are written to the blank disk at the time of manufacture and read by the optical disk drive during the OPC process. During the OPC process, operations of a CD-R drive differ from a CD-RW drive mainly in that erasing power is irrelevant for the CD-R drive.
Conventional optical disk drives determine the optimal recording and erasing powers using circuitry incorporating Eqn. 1 and Eqns. 2. However, this method is highly sensitive to measurement noise. Specifically, as the gamma curve is related to measured modulation amplitudes by a differential function (Eqn. 1) and modulation amplitude measurement is susceptible to noise, the calculated gamma curve can contain serious errors. Errors in the gamma curve show up in determination of the target power and the recording and erasing powers. Curve fitting is usually employed to reduce the effects of this problem, however, at the cost of program space and computation time. Higher order curve fitting yields better results, but at the cost of optical drive processing resources and time. In conventional drives, lower order curve fitting is preferred, with the risk of determining erroneous recording and erasing powers being accepted for sake of recording speed. Another method of reducing errors in the gamma curve is to use a low pass filter to smooth the measured modulation signals. Generally, curve fitting and low pass filter smoothing require additional hardware and additional costs. Hence, the conventional method for determining optimal recording and erasing powers for an optical disk drive is inefficient.